1. Field of the invention
The present invention relates to a gas sensor capable of detecting a concentration of a specific gas contained in measuring gases such as exhaust gas discharged from internal combustion engines such as engines of motor vehicles, in particular, relates to a structure of a protection cover capable of protecting a gas sensor element in a gas sensor.
2. Description of the Related Art
A recent vehicle is equipped with a gas sensor including a gas sensor element capable of detecting a concentration of a specific gas contained in measuring gases, for example, oxygen contained in exhaust gas emitted from an internal combustion engine of a vehicle. An electric control unit (ECU) mounted on such a vehicle receives a detection signal regarding the concentration of oxygen and other gas transferred from the gas sensor, and calculates an air-fuel ratio of the vehicle based on the concentration of oxygen detected. The ECU then controls the combustion or burning operation of the internal combustion engine based on the calculated air-fuel ratio. In general, the gas sensor has the gas sensor element which is inserted and placed in a housing thereof. The gas sensor is fixed to the wall of an exhaust gas flowing passage. A front end part of the gas sensor element in the gas sensor projects to or is exposed to gas flow in the exhaust gas flowing passage. In the gas sensor, the gas sensor element is covered with a protection cover fixed to the housing so as to protect it from the exhaust gas which is flowing through the exhaust gas flowing passage.
On starting the internal combustion engine under a low temperature environment, the thermal energy of moisture contained in the exhaust gas is absorbed by the cooled exhaust gas flowing passage or pipe and the moisture in the exhaust gas is thereby condensed. The condensed moisture becomes drops of water (hereinafter, referred to as “water-drops”). The water-drops move in the exhaust gas flowing passage or pipe without vaporization to the gas sensor. The exhaust gas containing water-drops then enters into the gas sensor. In this case, there is a possibility of contacting the water-drops contained in the exhaust gas as measuring gases onto the surface of the gas sensor element in the gas sensor.
On measuring the concentration of oxygen and other gases contained in the exhaust gas, the gas sensor element composed mainly of a solid polymer electrolyte membrane is heated at a temperature of more than 400° C. by a heater and the like in order to keep its optimum activation state. In the optimum activation state of the gas sensor element, there is a possibility of generating cracks in the gas sensor element by thermal shock when water-drops enter into the gas sensor and then adhered onto the surface of the gas sensor element.
Because there is a need to have a gas sensor of a high responsiveness in order to enhance the accuracy of controlling the combustion of the internal combustion engine, it is necessary to suck or introduce the exhaust gas into the gas sensor as fast as possible, in order to achieve the quick responsiveness of the gas sensor. There is therefore a need that the cover body covering the gas sensor element has different types of characteristics, the water proof capability (anti-adhesion capability of water-drops) and the high responsiveness, which are inconsistent with to each other.
Japanese patent laid open publication JP 2004-245103 has disclosed a gas sensor composed mainly of a cover body and a gas sensor element. The cover body has a double cylindrical construction composed of an inner cover and an outer cover which has a different radius placed in concentric configuration. In such a gas sensor, each of the inner cover and the outer cover has gas introduction holes, in order to increase its responsiveness, through which measuring gases are introduced into the gas sensor. A gap or a clearance between the inner cover and the outer cover in the gas sensor is set within a specified constant range in order to prevent entering water-drop components contained in the measuring gases through the side surface of the cover body.
The inventor according to the present invention has proposed two types of gas sensors, which have been disclosed in JP 2006-124074 and JP 2006-199073, having a cover body of an improved water proof capability.
FIG. 10 shows a configuration of the gas sensor 1b disclosed in the former proposal JP 2006-124074, and FIG. 11 shows a configuration of the gas sensor 1c disclosed in the latter proposal JP 2006-199073.
As shown in FIG. 10, the cover body of the gas sensor 1b accommodating and covering the gas sensor element 11 has a double cylindrical configuration composed mainly of an inner cover 12b and an outer cover 13b having a different radius. The inner cover 12b and the outer cover 13b are constructed in concentric configuration. In the gas sensor 1b, opening parts 123 are formed at the upper side surface of the inner cover 12b so that the opening direction of each opening part 123 turns upward from the outside to the inside of the inner cover 12b. An opening part 126b is formed at the middle part of a bottom surface 125b of the inner cover 12b. Further, a plurality of side opening parts 132 are formed at the upper side of the side surface of the outer cover 13b. Through the opening parts 132, the measuring gases such as exhaust gas are introduced into the gas sensor element 11. An opening part 133b is formed at the middle part of the bottom surface 135b of the outer cover 13b so that the opening part 133b is in concentric with the opening part 126b of the inner cover 12b. 
In the gas sensor 1b shown in FIG. 10, because each opening parts 123 in the side surface of the inner cover 12b is so formed that it turns upward, this configuration of the opening parts 123 prevents invading water-drops into the inner cover 12b, which is introduced with the exhaust gas through the opening part 132 formed in the side surface of the outer cover 13b. The water-drops contained in the exhaust gas fall to the bottom surface 125b through the inner wall of a radius-decreased part 124b formed at the front part of the inner cover 12b. The exhaust gas is finally discharged through the opening part 126b in the bottom surface 125b of the inner cover 12b to the outside of the gas sensor 1b. 
The opening part 126b in the bottom surface 125b of the inner cover 12b is formed in a same surface (see FIG. 10) of the opening part 133b in the bottom surface of the outer cover 13b, and protrudes toward the downward when compared with the opening part 133b in the bottom surface of the outer cover 13b in order to achieve a high responsiveness.
On the contrary, the cover body covering the gas sensor element in the gas sensor 1c shown in FIG. 11 has a double cylindrical structure in which the inner cover 12c and the outer cover 13c have a different radius and configured in concentric configuration. In the gas sensor 1c, a side gap 200c is formed between the outer circumference surface of the inner cover 12c and the inner circumference surface of the outer cover 13c, and the opening part 123 at the upper part of the side surface of the inner cover 12c turns upward from the outer part of the inner cover 12c toward the inner part of the inner cover 12c. The opening part 126c is formed at the middle part of the bottom surface 125c of the inner cover 12c. A plurality of opening parts 132 is formed at the upper part of the side surface of the outer cover 13c, through which measuring gases such as exhaust gas are introduced into the side gap 200c. 
The bottom surface 134c of the outer cover 13c is placed in position below the bottom surface 125c of the inner cover 12c. A bottom gap 201c is formed between the bottom surface 125c of the inner cover 12c and the bottom surface 134c of the outer cover 13c. A plurality of opening parts 133c is formed at the bottom surface of the outer cover 13c which is positioned outward from the opening part 126c in the bottom surface of the inner cover 12c. 
In the gas sensor 1c having the above configuration, the measuring gases such as exhaust gas are introduced or sucked into the side gap 200c through the opening parts 132 formed in the side surface at the upper part of the outer cover 13c, and then flow to the opening part 133c in the bottom surface of the outer cover 13c. Because the opening parts 123 in the inner cover 12c open upward to the inner cover 12c, even if water-drop is contained in the measuring gases such as exhaust gas, the water-drop does not enter the opening parts 123 formed in the side surface of the inner cover 12c and only the measuring gases such as exhaust gas are introduced into the inside of the inner cover 12c. Hence, the water-drop contained in the measuring gases such as exhaust gas introduced through the opening parts 132 in the side surface of the outer cover 13c is quickly discharged to the outside of the gas sensor 1c through the opening part 133c in the bottom surface of the outer cover 13c. It is thereby difficult to adhere or contact the water-drop contained in the measuring gases to the gas sensor element 11 for detecting the gas concentration.
Still further, because the opening part 126c in the bottom surface of the inner cover 12c does not directly contacts to the opening part 133c in the bottom surface of the outer cover 13c, even if a water-drop enters the inside of the outer cover 13c through the opening parts 133c in the bottom surface of the outer cover 13c, the water-drop does not directly enters the inside of the inner cover 12c and the water-drop is evaporated in the bottom gap 201c formed between the bottom surface 125c of the inner cover 12c and the bottom surface 134c of the outer cover 13c. This configuration can avoid the occurrence of entering water-drop to the inside of the gas sensor element 11.
However, in each of the cover bodies composed mainly of the inner over and the outer cover disclosed in JP 2004-245103 and JP 2006-124074 shown in FIG. 10, because the opening part in the bottom surface of the outer cover is constructed in concentric with the opening part in the bottom surface of the inner cover, and the opening part in the bottom surface of the inner cover is exposed to the measuring gases such as exhaust gas, it is difficult to completely prevent the invasion of water-drop into the inner cover through the opening part formed in the bottom surface of the inner cover according to a setting angle or an inclination angle of the gas sensor. In particular, as shown in FIG. 10, when the opening part 126b in the bottom surface of the inner cover 12b and the opening part 133b in the bottom surface of the outer cover 13b are in a same surface, the water-drop stayed in the edges of the opening part 133b in the bottom surface of the outer cover 13b in addition to directly invading water-drops contained in the exhaust gas as measuring gases enter into the inside of the inner cover 12b, and are contacted or adhered to the surface of the gas sensor element 11 when the exhaust gas fast flows.
On the contrary, the gas sensor 1c having the configuration shown in FIG. 11 has a high performance of preventing the invasion of water-drop into the gas sensor element 11. Further, the gas sensor 1c has a highly step response capability which is approximately equal to that of the gas sensor 1b having the configuration shown in FIG. 10 when the flowing speed of the exhaust gas is high. However, the configuration of the inner cover 12c of the gas sensor 1c generates a complicated spiral vortex in the exhaust gas as measuring gases, and generates a negative pressure of a small magnitude, not a large magnitude, which can be generated by the exhaust gas flow in the bottom gap 201c formed at the bottom surfaces 125c and 134c which is necessary for discharging the exhaust gas from the opening part 126c in the bottom surface of the inner cover 12c. Further, when the flow speed of the exhaust gas is low, for example, on starting an internal combustion engine or when a motor vehicle moves at a low speed, the step response capability and a frequency response of the gas sensor 1c become low when compared with those of the gas sensor 1b shown in FIG. 10.